Youthful Star Sprouts Planets Early

by Linda Vu

Jet Propulsion Laboratory News Feature

2007 November 28

A stellar prodigy has been spotted about 450 light-years away in a system called
UX Tau A by NASA's Spitzer Space Telescope. Astronomers suspect this system's
central sun-like star, which is just one million years old, may already be
surrounded by young planets. Scientists hope the finding will provide insight
into when planets began to form in our own solar system.

"This result is exciting because we see a gap, potentially carved out by planets,
around a dusty sun-like star. In almost all other star systems of this age,
wetypically see a primordial disk – a thick disk of dust, without any clearings,"
said Catherine Espaillat, a graduate student at the University of Michigan, Ann
Arbor.

Prior to the Spitzer observations, Espaillat and her teammates knew that a
sun-like star sat at the center of UX Tau A. Now, using the telescope's infrared
spectrometer instrument, they have discerned details about the dusty disk
swirling around the central star.

Such dusty disks are where planets are thought to be born. Dust grains clump
together like snowballs to form larger rocks, and then the bigger rocks collide
to form the cores of planets. When rocks revolve around their central star, they
act like cosmic vacuum cleaners, picking up all the gas and dust in their path
and creating gaps.

Spitzer saw a gap in UX Tau A's disk that extends from 0.2 to 56 astronomical
units (an astronomical unit is the distance between the sun and Earth). In our
solar system, this gap would occupy the space between Mercury and Saturn.
Espaillat notes that the formation of one or more planets could be responsible
for carving out the gap.

Although gaps have been detected in disks swirling around young stars before,
Espaillat notes that UX Tau A is special because the gap is sandwiched between
two thick disks of dust. An inner thick dusty disk hugs the central star, then,
moving outward, there is a gap, followed by another thick doughnut-shaped disk.
Other systems with gaps contain very little to no dust near the central star. In
other words, those gaps are more like big holes in the centers of disks.

Some scientists suspect that these holes could have been carved out by a process
called photoevaporation. Photoevaporation occurs when radiation from the central
star heats up the gas and dust around it to the point where it evaporates away.
The fact that there is thick disk swirling extremely close to UX Tau A's central
star rules out the photoevaporation scenario. If photoevaporation from the star
played a role, then large amounts of dust would not be floating so close to the
star.

"This finding definitely affects the way astronomers look at planet formation.
Spitzer's infrared spectrometer was able to see a gap in this system, but
future, more sensitive telescopes maybe able to search for Earth-like planets in
UX Tau A," said Espaillat.

Her paper will be published in the December 2007 issue of Astrophysical Journal
Letters. Other authors on the paper include Nuria Calvet, Jesus Hernández and
Lee Hartmann, also from the University of Michigan; Paola D’Alessio of the
Universidad Nacional Autónoma de México, Michoacán; Chunhua Qi of the
Harvard-Smithsonian Institute for Astrophysics, Cambridge, Mass.; Elise Furlan of
the NASA Astrobiology Institute at the University of California at Los Angeles;
and Dan Watson of the University of Rochester, N.Y.